1. Field of the Invention
The invention relates to a method for forming compactible ceramics and for forming ceramics with improved compression, flexural and fracture strength, and more specifically, this invention relates to a method for using a phosphate binder with enhanced binding characteristics in high waste loading scenarios which is compactible in the paste stage and which can be used with fibers to improve the ceramics"" strength.
2. Background of the Invention
The effective sequestration and disposition of waste oils, bulky waste forms and other unwieldy objects continues to elude disposal researchers. Typical concretes and ceramics, the later of which are described in U.S. Pat. Nos. 5,846,894, 5,830,815, 5,645,518, and 6,204,214, and incorporated herein by reference, have high compression strengths. However, these materials exhibit comparatively poor flexural and fracture properties. This leads to crack propagation, particularly when attempting to macroencapsulate large-size objects. Compactibility of these materials also is lacking.
To macroencapsulate large-size objects, better fracture toughness is needed to avoid crack propagation. For cementing lateral junctures in multilateral oil well completions, improved flexural strength is needed. To arrest surface crack propagation, fiber reinforcement is needed.
The use of glass fiber as a strengthening additive in cement causes problems. Cement is very alkaline and glass fibers deteriorate in such alkaline environments. This leads to weakening of the composite structure. Some common solutions are to over-engineer the composite to compensate for the eventual degradation and loss of strength, to use alkali resistant glass fibers, and to use coatings such as polyvinyl chloride (PVC) over the glass fiber to protect it from the alkaline environment. All of these solutions lead to higher costs.
Ceramic systems leading to highly ductile waste forms remain elusive. For example, in the ceramic formation reaction disclosed in U.S. Pat. No. 5,830,815, and given by Equation 1, infra.
MgO+H3PO4+2H2Oxe2x86x92MgHPO4xc2x73H2Oxe2x80x83xe2x80x83Eq. 1 
102.44 Kilojoules per mole of heat are released. This high amount of heat results in too rapid ceramic product formation (leading to brittleness) for any practical use. Also, some materials for disposal prove too soluble in the very low pH environs in which the phosphate ceramics disclosed in the ""815 patent operate. Alternatively, solids of low solubility (pKsxcx9c15 to 25) are not soluble enough in the solutions utilized in the ""815 patent.
U.S. Pat. No. 5,846,894 issued to Singh et al. on Dec. 8, 1998 discloses a method to produce phosphate bonded structural products from high volume benign wastes.
U.S. Pat. No. 5,678,234 issued to Colombo, et al. on Oct. 14, 1997 discloses an encapsulation method utilizing a modified sulfur cement at elevated temperatures, and glass or other fibers for enhancement of the compressive and tensile strength.
None of the aforementioned patents teaches a method for the reduction of volume during stabilization of solid powdered wastes. In addition, none of these patents provides a method for the stabilization of near insoluble oxides.
None of the aforementioned patents teaches a method for enhancing the flexural and fracture toughness of the structural products via a truly homogeneous ceramic-fiber composite.
None of the aforementioned patents even contemplates using a dispersant for fiber additives for strength enhancement.
A need exists in the art for a method to produce superior compactible structural products with enhanced flexural and fracture toughness. The method should result in a ceramic which can be compressed while it is still putty-like, i.e., before it sets completely. The method also should result in the formation of a durable and chemically stable ceramic which can be utilized to sequester hard-to-contain wastes. Finally, the method should utilize inexpensive and commonly available reactants at ambient temperatures to produce low cost ceramics.
An object of the present invention is to provide a method for producing chemically bonded phosphate ceramics (CBPCs) that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a ceramic capable of encapsulating very low solubility metal oxides. A feature of the invention is that the oxides do not need to be calcined prior to encapsulation. An advantage is that an encapsulation process utilizing a ceramic uses much less energy and, accordingly, is less expensive.
Still another object of the present invention to provide a method for producing ceramics which have enhanced flexural and fracture toughness. A feature of the invention is that fibers being evenly dispersed throughout the ceramic binder enhances the flexural and fracture toughness of the binder. An advantage of this feature is that it minimizes leakage of encapsulated hazardous wastes.
Yet another object of the present invention is to provide a method for producing ceramics which can encapsulate wastes that contain nonpolar, or oil-based fluids. A feature of this invention is that a magnesium dihydrogen phosphate (MHP)-based binder can effectively encapsulate wastes having trace amounts of oil. An advantage of this feature is that, at present, there is not any effective means for encapsulating petroleum fluid-tainted wastes.
Another object of the present invention is to provide a method for producing glass fiber-reinforced ceramics. A feature of the invention is that phosphate-based ceramics are homogeneously mixed with the fibers to produce a ceramic structure containing fibers dispersed throughout the structure. An advantage of the method is that it provides an acidic- to neutral-pH environ favorable to the glass fibers. Therefore, corrosion of the glass fibers is minimized, and structural integrity of the resulting structures is maximized for periods of time heretofore not attainable.
Still another object of the present invention is to provide a method for the effective dispersal of fiber additives in a ceramic binder. A feature of the invention is that monopotassium phosphate is utilized as a dispersant to prevent aggregation of fibers into strands and bunches. An advantage of this feature is that the flexural strength of the resulting structure is typically twice that of cements.
Yet another object of the present invention is to provide a method which produces compactible ceramics. A feature of the invention is the use of magnesium dihydrogen phosphate as a binder. An advantage of the method is that it allows for compression of the ceramic to a volume 40% less than the starting volume of the reaction slurry.
Briefly, the invention provides a room temperature process for producing a compactible ceramic from powders, the process comprising combining MgO and magnesium dihydrogen phosphate dihydrate to create a dry homogeneous mixture; and contacting the mixture with water to form a slurry.
Also provided is a process for strengthening phosphate ceramics, the process comprising adding fibers to the initial ceramic mixture to create a homogeneous composite substrate.